I think I've changed the batteries in the Trv once so far, and I've had it for 6 months.
Roughly how long does the battery last do you reckon?
The Tado App tells you when the batteries are low/have run out, so you don't really run into that problem. That and if you buy some rechargeable batteries... Problem solved.
I suspect you will need a smart thermostat kit and an extension kit. The Tado customer service is very good - if you contact them and provide the manufacture details of your current system then they should be able to advise.
Our whole house is Tado'd; 14 TRVs, smart thermostat and extension kit. We used to have Hive and I've found Tado to be infinitely better. - Its obviously more suited to larger homes where the one-temperature-suits-all mode doesn't work (hive/nest). With the TRVs, you only heat the rooms you use, for example; Through the night, only our bedrooms are heated. In the morning, the kitchen and breakfast rooms are heated, through the day the dining and games rooms are heated, in the evening it's the livingroom and kids bedrooms etc. etc. On top of that, it has geo-location fencing to turn off/ switch on the heating when you (or anyone else logged in) are leaving/returning from a custom-set boundary. I've easily saved hundreds in the past year compared to our gas bill before installing it which has paid for the system and more. I researched the options for months before choosing, Tado's promise of Apple Homekit support swung it for me and since they've finally made good on that, I've never looked back. I also installed it myself, I have no clue when it comes to boilers but just like Hive, it's a simple plug-and-play install. As for the deal, it's a good discount but still far more than what I paid. Keep an eye on ebay, sometimes installers flog unused/unwanted sets really cheap.